Some proposals have conventionally been made on retroreflective articles including cube corner retroreflective elements having excellent retroreflective efficiency and entrance angle characteristic.
For example, U.S. Pat. No. 1,591,572 (Patent Document 1) by Stimson discloses hexagonal cube corner retroreflective elements. However, there is no description as to what shape of elements is advantageous for obtaining excellent entrance angle characteristic, observation angle characteristic and rotation angle characteristic.
U.S. Pat. No. 3,417,959 (Patent Document 2) and U.S. Pat. No. 3,922,065 (Patent Document 4) by Schultz disclose a method (pin bundling method) of forming a prism at a tip of a metal pin and bundling a number of such pins to form a prism assembly. This method for forming a prism assembly is suitable for producing relatively large prisms, but is not practical when it is necessary to form 2,000/cm2 or more microprisms, for example.
In addition, U.S. Pat. No. 3,458,245 (Patent Document 3) by Stanley discloses retroreflective prisms in which at least two surfaces, or preferably four or more surfaces are arranged alternately forming an obtuse angle and an acute angle.
Furthermore, U.S. Pat. No. 3,924,929 (Patent Document 5) by Holmen et al. also discloses a retroreflective article formed of a repeated pattern of units in which hexagonal prisms are hermetically sealed.
U.S. Pat. No. 4,066,331 (Patent Document 6) by Lindner also discloses a retroreflective article in which hexagonal prisms that are different for each row are arranged.
U.S. Pat. No. 4,073,568 (Patent Document 7) by Heasley also discloses a retroreflective article in which one type of hexagonal prisms are arranged in a repeated pattern.
Similarly, U.S. Pat. No. 4,189,209 (Patent Document 8) by Heasley also discloses a retroreflective article in which two types of hexagonal prisms having different thickness are arranged in a repeated pattern.
Furthermore, U.S. Pat. No. 6,114,009 (Patent Document 9) by Smith discloses a mold suitable for forming cube corner retroreflective sheeting, a method for producing the mold and retroreflective sheeting formed using the mold, and particularly discloses a mold formed of a plurality of thin laminae and a method for producing the mold.
Furthermore, Japanese Utility Model Application Laid-Open No. S63-109233 (Patent Document 10) by Kato discloses a reflective article including a first reflecting part having reflective performance with respect to light incident at a critical angle or larger from the left and a second reflecting part having reflective performance with respect to light incident at a critical angle or larger from the right.
U.S. Pat. No. 6,120,280 (Patent Document 11) and U.S. Pat. No. 6,010,609 (Patent Document 12) by Mimura et al. disclose a design of hexagonal cube corner retroreflective elements having an asymmetrical shape in which optical axes are tilted leftward and rightward of the elements and a method of making the element.
It is stated therein that according to such hexagonal cube corner retroreflective elements having an asymmetrical shape in which optical axes are tilted leftward and rightward of the elements, an improved entrance angle characteristic is achieved in the tilt direction of the two optical axes. However, in the elements disclosed in these documents, excellent improvement in rotation angle characteristic cannot be achieved.
On the other hand, U.S. Pat. No. 6,318,866 (Patent Document 13) by Mimura et al. discloses various proposals on improvement of observation angles.
This Patent Document 13 discloses that the observation angle characteristic can be improved by making side surfaces that face each other of a pair of triangular pyramidal cube corner retroreflective elements have different shapes.